Site description Lake Natron is a shallow soda-lake in a closed basin on the floor of the Eastern Rift Valley. It extends 58 km south from the Kenyan border with a mean width of 15 km. Although its surface area can reach 850 km², the maximum depth is only 50 cm. There are considerable seasonal fluctuations in surface area between the middle of the dry season (November–December) and the end of the rains (April–May). The water is highly saline with chloride concentrations reaching 65,000 mg/litre and is unsuitable for direct human and livestock use. The lake’s principal inflow is the perennial Ewaso Ngiro river which rises on the Mau Escarpment in Kenya and flows southwards along the eastern edge of the Nguruman Hills. Seasonal drainage from within Tanzania includes major rivers from the Loita Hills (rising in Kenya) and Longido mountains in the north-west, the Gol mountains in the west, the Ngorongoro Highlands to the south and minor streams from Mount Gelai in the south-east.The surrounding land is dry bush dominated by Acacia thorn-trees, inhabited by pastoralist Masai. There is some seasonal cultivation along the riverbanks and a small settlement in the south associated with a minor soda-extraction plant and a few small tourist camps. Otherwise the general area is sparsely populated, the lack of fresh water in the dry season controlling the populations of both man and livestock. Engaruka is a shallow depression in the Rift Valley 50 km north-north-east of Lake Manyara and 58 km south south-east of Lake Natron. The centre of the depression is a semi-permanent lake covering in excess of 400 ha. Surrounding this lake is a swamp that can extend for up to 3 km, especially to the north and south-east. Beyond this lies a shallow plain which is occasionally inundated over a further 4,000 ha. During dry years the area is a rather featureless plain fringed with low acacia scrub and heavily grazed by herds of Masai cattle and goats.
Key Biodiversity See Box and Table 3 for key species. Lake Natron is the most significant and regular breeding site for the majority of the world population of Phoenicopterus minor. The lake also seasonally holds significant numbers of waterbirds, many of which would probably meet 1% thresholds, but for which data are lacking. Among these are likely to be Phoenicopterus ruber, Plegadis falcinellus, Charadrius pallidus and Calidris minuta. Of particular note is a count of 148,000 Ciconia abdimii in 1995 at Engaruka, and an estimated 60,000 were seen here in January 1998. A number of Somali–Masai biome species reach their western limits in this area, including Emberiza poliopleura, Mirafra poecilosterna and Apalis rufifrons.
Non-bird biodiversity: There are considerable numbers of ungulates in the ecosystem, including Oryx gazella, while Lake Natron is the northern limit for wildebeest Connochaetes migrating north from Tarangire National Park.